Kate Becker: Astronomers identify a new kind of 'mini-supernova'

By Kate Becker, For the Camera

Posted:
04/04/2013 11:05:34 PM MDT

Updated:
04/04/2013 11:07:52 PM MDT

Kate Becker

What is your favorite kind of supernova?

First, there's the core-collapse variety: This is what you get when you take a single, hulking star -- 10 or more times as massive as our sun -- and let it burn until it begins to run out of hydrogen and starts fusing heavier elements.

Eventually, it can't produce enough energy to hold itself up against gravity, and it dramatically collapses, sending out a shockwave that blows off the outer layers and leaves behind only a small, dense core called a neutron star. The most massive such stars end up as black holes.

Then there are Type Ia supernovas. At this point, you might be wishing that astronomers could loosen up a bit with their naming schemes. Since that probably won't happen anytime soon -- this is the same community, after all, that brought you the Very Large Array and the Very Large Telescope -- let's take matters into our own hands and call these the "It Takes Two" supernovas, because it takes two stars to make them.

In a binary star system -- that is, a system in which a pair of stars orbit around a common point called the center of mass -- one star is inevitably a little bigger than the other one. That bigger star will evolve faster than its companion, burning through its hydrogen while its petite partner still has fuel to spare, then ballooning up into a red giant before deflating into the cooling ember known as a white dwarf.

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Meanwhile, the less massive star is inching its own way through the stages of stellar evolution. By the time it puffs into a red giant, its companion is already a white dwarf. If the binary orbit is sufficiently cramped, the gravity of the white dwarf will start tugging gas off of the red giant, pulling at it like a thread on a frayed sweater.

Eventually, the white dwarf will take on too much additional mass, overtopping the threshold of 1.4 times the mass of our own sun and setting off a nuclear chain reaction that ends in the total destruction of the white dwarf. From our comfortable perch back on Earth, we see the star suddenly brighten; at its peak, the explosion will put out five billion times as much light as our sun.

There are probably other ways to make an "It Takes Two" supernova, but this recipe is particularly important to astronomers because every supernova it churns out is just about as intrinsically bright as the next. Of course, we don't see them all as equally bright. The closer they are, the brighter they appear -- which means that astronomers can use them as cosmic yardsticks. In fact, it was by measuring the brightness of Type Ia supernovas that astronomers realized that our universe's expansion is accelerating.

Now, astronomers think they've identified a new kind of supernova: a "mini-supernova" they've labeled Type Iax. These supernovas are much dimmer than typical "It Takes Two" explosions, and they don't seem to explode as powerfully, but a close analysis of their light shows that, just like Type Ia supernovas, they probably come from binary systems containing a white dwarf. So why are they so (comparatively) anemic?

That's what scientists are now trying to figure out. They think that Type Iax supernovas probably start out like their "It Takes Two" cousins, but end differently: The white dwarf apparently survives the mini-supernova rather than being destroyed completely. So far, there are 25 examples of this new kind of supernova, but astronomers are optimistic that a telescope called the Large Synoptic Survey Telescope, planned to come online in about a decade, will turn up thousands more.

So which is your favorite? The most massive, the most useful or the newest addition to the menu? I'll pick any one that's comfortably far away: Distant enough that it doesn't pose any threat to Earth, but close enough to give us a precious window into the last moments of a star's life and, perhaps, the fate of the universe itself.

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